Polymeric materials are generally considered useful as viscosification agents when dissolved in an appropriate solvent system. The major reason for this viscosity enhancement is due to the very large dimensions of the individual polymer chain as compared to the dimension of the single solvent molecules. Any increase in the size of the polymer chain will produce a corresponding enhancement in the viscosity of the solution. This effect is maximized when the polymer is dissolved in a "good" solvent. Therefore, in general, a hydrocarbon soluble polymer is useful for thickening hydrocarbon solvents, while a water soluble polymer is appropriate for increasing the viscosity of aqueous systems. With regard to aqueous solutions, water soluble nonionic polymers and low charge density sulfonated ionomers are quite useful in this regard and are commonly used materials. However, the solution properties of the former family of materials are controlled primarily through modification of the molecular weight of the polymer and through changes in the level of dissolved polymer. These materials become especially effective at concentrations where the individual polymer chains begin to overlap. This "transition" is commonly referred to in the literature as the chain overlap concentration or simply C*. It should be noted that in most nonionic polymers of commercial interest, a relatively large amount of polymer is required prior to reaching C*. Therefore, this approach is undesirable from an economic viewpoint. Moreover, the rheological properties of many of these nonionic systems have been published. The results of these studies show that, in general, these solutions are shear thinning over all shear rates investigated.
It should be noted that polyelectrolytes are very useful and the most commonly used materials. However, the solution properties of these materials begin to deteriorate as low molecular additives (i.e., acids, bases or salts) are dissolved in the solution. These additives screen the charges that are fixed along the chain backbone which results in a decrease in the dimensions of the polymer molecule. The viscosity diminishes as long as the chain continues to shrink.
It has been found previously (U.S. Pat. Nos. 4,460,758 and 4,540,496), for example, that ampholytic ionomers, composed of a nonstoichometric ratio of anionic and cationic monomer units, can be quite useful in thickening a broad variety of aqueous solutions, as, indeed, is necessary for the appropriate rheological control of well control and workover fluids, completion fluids, drag reduction, enhanced oil recovery, acid gelation and the like. More importantly, these polymeric materials possess markedly higher viscosity in acid, base or brine solutions than in the corresponding fresh water systems. Even more interesting is the finding that these polyampholytes show a corresponding enhancement in viscosity as the level of dissolved acid, base or salt is increased. These rheological properties are unexpected for an ion-containing water soluble polymer since the general tendency of these polymer types is to show a strong decrease in thickening efficiency.
Furthermore, in recent years, interpolymer complexes have received considerable attention in the literature due to their interesting and unique properties and their similarity to certain biological systems. In most instances, these complexes are formed by intimately mixing aqueous solutions containing high-charge density polyelectrolytes possessing opposite charge. When these polymer molecules meet in solution, the interaction between oppositely charged sites will cause the release of their associated counterions forming the complex. The counterions are now free to diffuse into the bulk solution. Normally, phase separation occurs on prolonged standing.
Significantly, these interpolymer complexes are normally 1:1 compositions of the polyanions and polycations. The neutrally charged complex swells and absorbs both water and electrolyte when immersed in aqueous electrolyte solutions of increasing concentration. Nonstoichometric complexes, on the other hand, behave as a rather conventional ion-exchange resin in electrolyte solutions. In these specific instances, dissolution does not occur in these high ionic strength media nor in a salt-free environment. In fact, the latter complex can be cold-drawn and deformed as easily as a conventional thermoplastic resin. However, A. S. Michael (I&EC, 57, 32 [1965] has shown these "intractable polysalt precipitates" can dissolved (i.e., codissolved without reaction) in selected ternary solvent mixtures comprising water, water-miscible organic solvent (e.g., acetone) and a strongly ionized inorganic electrolyte (e.g., NaBr). These solutions do yield a homogeneous, transparent viscous solution. It is further noted that a soluble complex is formed in an aqueous solution with the addition of an excess high-charge density polyelectrolyte (E. Tsuchida et al., J. Polymer Sci., Polym. Chem. Ed., 10. 3397 (1972). To the authors, knowledge, few studies have focused on the viscosification aspects of these materials and even less work appears on low-charge density polycomplexes. In these studies, viscosity is used only as a tool to study the extent and mechanism of complex formation.
In previous U.S. patents (U.S. Pat. Nos. 4,615,393 and 4,665,115) it is reported that low-charge interpolymer complexes are soluble and effective in viscosifying aqueous solution systems. More importantly, these complexes possess a substantially higher viscosity than the corresponding individual low-charge density copolymer components. As detailed earlier, these characteristics are unexpected since high-charge density complexes are insoluble in these conventional solution systems. Therefore, it is anticipated that few detailed rheological studies of these latter materials appear in the literature. In particular, shear rate measurements are markedly absent.
Even more interesting is the unique and unexpected result that these soluble interpolymer complexes are very effective in enhancing the viscosity of aqueous solutions over a broad shear rate range. With these unique polymeric materials, dilatant behavior, i.e. shear thickening, occurs in aqueous fluids which are of extreme technological utility. It is further noted that under the identical experimental conditions, the viscosity of the individual copolymer components show the anticipated shear thinning behavior.
The instant invention teaches that a novel family of alkoxypropyl quaternary monomers (derived from glycidal alkyl ether reacting with dimethylaminopropyl methacrylamide) to form a unique family of cationic-hydrogen bonding type hydrophobically associating copolymers. These copolymers are found to be useful in thickening in a very effective manner both fresh water and brine solutions. Even more important is that these copolymers have novel, improved and quite different solution properties as compared to conventional nonionic polymers and polyelectrolytes. These copolymers are based on, but not limited to, the incorporation of the above cationic monomers into an acrylamide backbone structure.
It is well known that polyacrylamide and hydrolyzed polyacrylamide are water soluble polymers that have been previously disclosed in the literature and have found application in the viscosification of aqueous solutions. This is achieved through a combination of high molecular weight and chain expansion due to repulsion of pendant ionic groups along the polymer chain. However, high molecular weight polymers present well known difficulties in manufacture and subsequent processing because of their irreversible degradation when exposed to conditions of high shear such as would be obtained in the usual stirring devices. Moreover, the presence of pendant ionic groups leads to solution properties which are markedly influenced by the presence of dissolved cations. In particular, the viscosity of solutions of these polymers usually decreases strongly upon increasing concentrations of brine.
This invention teaches that an alternative means for providing polymers which viscosify water or brine at low concentrations. This method relies on the incorporation of a small amount of hydrophobic groups into a polymer with a water soluble backbone. These hydrophobic groups tend to associate with one another in an aqueous solutions, and when the association occurs intermolecularly, the solution viscosity may be increased relative to the polymer without the hydrophobic side groups. An additional benefit is that the solution viscosity is relatively insensitive to salts because the hydrophobic groups are not ionic. A further benefit is the ability to easily incorporate these hydrophobic-type monomers into the polymer structure due to the water solubility of these said monomers. The cationic and hydrogen-bonding ability of these monomers facilitate water soluble and as a result, polymerizability.
The synthesis of copolymers composed of water soluble and water insoluble monomers presents difficulties. In order for polymerization to be effected, the monomers must obviously come into close proximity to one another. A variety of processes based upon prior rt could conceivably achieve this, but have serious deficiencies, necessitating the instant invention. For example, simply dispersing the water insoluble monomer as fine particles in the aqueous medium, while dissolving the water soluble monomer in water would result in poor incorporation of the water insoluble monomer and would lead to a heterogeneous product of particles dispersed in the predominantly water soluble polymer. This would therefore require the extra step of separating the unreacted monomer particulates from the reaction product.
Conventional emulsion polymerization, which uses a surfactant to disperse the water insoluble monomer into the aqueous medium containing the dissolved water soluble monomer, has other disadvantages. In this process, the bulk of the water insoluble monomer is contained initially in droplets which are at least one micron in diameter. These droplets must be stabilized against coalescence by a combination of agitation and added surfactant. The product copolymer is usually in the form of particulates with diameters on the order of 500 to 2000 .ANG. in diameter.
Alternatively, both monomers may be dissolved in a solvent or solvent mixture having properties intermediate between water and a hydrocarbon solvent. Although this undoubtedly allows the comonomers to come into close proximity to one another, since the dispersion is on a molecular scale, this process presents other difficulties. For example, often the copolymer is insoluble in the mixed solvent which is capable of solubilizing the monomers. This leads to precipitation of the copolymer when it has a molecular weight which is still too low to produce efficient viscosification. The reaction product is usually heterogeneous which therefore requires a disadvantageous additional processing step. Furthermore, the water miscible solvents such as alcohols, acetone, ethers and acetic acid are fairly good chain transfer agents and when used in reasonable quantities would lead to decreased molecular weights and hence poor viscosification efficiency.
It should be noted in this regard that the use of hydrophobic groups on water soluble polymers to enhance the rheological properties of water based fluids has been described. One approach to provide polyacrylamide based systems containing hydrophobic groups is described by Bock, et al., U.S. Pat. Nos. 4,520,182 and 4,528,348. Water soluble acrylamide copolymers containing a small amount of oil soluble or hydrophobic alkylacrylamide groups were found to impart efficient viscosification to aqueous fluids. Landoll, U.S. Pat. No. 4,304,902, describes copolymers of ethylene oxide with long chain epoxides which also required relatively large polymer concentration (approximately 1%) for thickening water and required surfactants for solubility due to irregularities in the polymerization. In a related case, U.S. Pat. No. 4,428,277, modified nonionic cellulose ether polymers are described. Although these polymers show enhanced viscosification relative to polymers not containing hydrophobic groups, the viscosification efficiency was very low, requiring 2 to 3 weight percent polymer to provide an enhancement. The use of surfactants to enable solubility and, in turn, viscosification, by a water soluble polymer containing hydrophobic groups is described by Evani, U.S. Pat. No. 4,432,881. The hydrophobic groups claimed are attached to the polymer via an acrylate linkage which is known to have poor hydrolytic stability. In addition, the need for a surfactant to achieve solubility and thickening efficiency should make such a system very salt sensitive, as well as very sensitive to small changes in surfactant and polymer concentrations. Emmons, et al., U.S. Pat. No. 4,395,524, teaches acrylamide copolymers as thickeners for aqueous systems. While these polymers possess hydrophobic groups they are prepared using alcohol containing solvent which are known chain transfer agents. The resulting polymers have rather low molecular weights and, thus, relatively high polymer concentrations are required to achieve reasonable viscosification of water based fluids.